Lasius niger has a reputation for being very stubborn when it comes to moving to a new tube and they are an ant species that can particularly well adapt to light, although the cotton may in fact still retain some moisture inside despite looking dry.

What you could do is to attach a second test tube to the first one using a straw to connect them both. The ants might then regard the new space as part of their nest and move in. If everything else fails you can also just gently tap them into a new tube (this is a lot easier when they don't have any brood but it's doable with brood as well.